Grinding fluid for grinding titanium metal and titanium metal alloys

ABSTRACT

The invention is a novel grinding fluid or coolant which improves the efficiency and economics of grinding titanium metal and titanium metal alloys when using grinding wheels made up of aluminum oxide or aluminum oxide containing abrasives. The grinding fluid is composed of salts of nitrous acid and formic acid dissolved in water thereby providing nitrite and formate ions. The use of the grinding fluid of this invention when grinding titanium and its alloys, in such precision grinding applications as surface grinding results in higher metal removal to wheel wear ratios and also results in more rapid removal of metal by permitting the use of higher infeeds than have been permissible with prior art grinding fluids.

United States Patent Inventors Appl. No. Filed Patented Assignee GRINDING FLUID FOR GRINDING TITANIUM METAL AND TITANIUM METAL ALLOYS 5 Claims, No Drawings 11.8. C1 51/281, 51/307, 5 1/309, 252/76 lint. Cl B2411 1/00, 824d 3/02 Field of Search 252/76; 51/295, 304,305,306, 281

References Cited UNITED STATES PATENTS 1,981,849 11/1934 Elliott et a1. 252/76 2,394,251 2/1946 Morris et al. 252/76 2,455,961 12/1948 Walker 252/76 3,020,140 2/1962 Bluth et a1. 51/306 Primary Examiner-Donald 1. Arnold Attorney-Rufus M. Franklin ABSTRACT: The invention is a novel grinding fluid or coolant which improves the efficiency and economics of grinding titanium metal and titanium metal alloys when using grinding wheels made up of aluminum oxide or aluminum oxide containing abrasives. The grinding fluid is composed of salts of nitrous acid and formic acid dissolved in water thereby provid ing nitrite and formate ions. The use of the grinding fluid of this invention when grinding titanium and its alloys, in such precision grinding applications as surface grinding results in higher metal removal to wheel wear ratios and also results in more rapid removal of metal by permitting the use of higher infeeds than have been permissible with prior art grinding fluids.

BACKGROUND OF THE INVENTION The invention relates to the precision grinding of titanium metal and high titanium metal alloys. More specifically the said invention relates to a grinding fluid which facilitates the difficult job of precision grinding the aforementioned metals by virtue of rendering the grinding operation more efficient and more economical.

When titanium and high titanium alloys first became of commercial significance, these materials were naturally subjected to the conventional machining techniques used for other established metals. Among these machining techniques to which the titanium and its alloys were subjected, was grinding, both rough grinding and precision grinding. It was soon discovered that these newer metals were by comparison, extremely difficult materials to grind. For example, when attempts were made to grind such high-strength titanium alloys like Ti-lSOA and RC130B using typical surface-grinding conditions such as: wheel speed 6,000 s.f.p.m., table speed 450 inches per minute, unit crossfeed 0.050 inches, unit downfeed 0.001 inches, the results were extremely poor and costly. The grinding wheels wore so fast that they would lose contact with the workpiece before the wheel had made a complete traverse across the workpiece; grinding with harder, stronger wheels did not improve the situation in that the wheel wear remained just as high as with the softer wheels and in addition, the workpiece now became badly burned, and severe loading of the wheel produced extremely poor finishes; all of this being contrary to normal experience. The grindability of the titanium at this stage was so bad that the maximum grinding ratio attainable was about 0.7. The grinding ratio or G- ratio is defined herein as cubic inches of material removed per cubic inch of wheel wear; the greater the G-ratio number, the smaller is the wheel cost per unit volume of metal removed. The degree of poorness of this grinding ratio can be readily appreciated from a consideration of the grinding ratios derived from the identical type of grinding of some high-grade hardened tool steels. For example high-chromium, high-carbon steels are successfully ground with a grinding ratio of 3 to 4; M-2 high-speed tool steels, with a grinding ratio of 4 to 12; and, plain-carbon and low-alloy tool steels with grinding ratios of from 40 to 80.

In depth studies of the anomolous behavior of grinding wheels towards titanium as alloys, soon provided some insight into the problem. It has long been known that the abrasive particles in a grinding wheel wear by two fundamental processes, both of which occur to a greater or lesser degree in almost all grinding operations. Normally, when an abrasive particle grinds or rubs on a metal surface attritious wear begins, that is a rubbing away of the sharp edges of the abrasive grain occurs. This creates a flat area on the grain particle which gradually becomes larger, until this surface area is so large that the frictional forces acting on the grain will become sufficient to cause the grain to either fracture or pullout of the face of the grinding wheel, in either case exposing new cutting edges to continue the grinding process. This combination of attritious wear and grain fracture or removal from the wheel face is essential to an effective grinding process. When the grinding of titanium and its alloys was studied from this point of view, it was found that under conventional grinding conditions, the attritious wear of the abrasive was extraordinarily high as compared to that when grinding more conventional metals. The attritious wear was so high that almost no grain fracture occurred. It was concluded that (l) the temperature between the abrasive grains and the titanium was significantly higher than between the more conventional metals and the abrasive grain during the grinding process, and (2) the titanium metal reacted more rapidly and severely with the abrasive grain than did other metals.

The aforementioned findings and conclusions led to the first major breakthroughs in the grinding of titanium. The temperature at the abrasive grain-titanium metal inner face was greatly reduced by reducing the peripheral speed of the grinding wheel from 6,000 s.f.p.m. down to 2,200 s.f.p.m.; this affected an increase in the grinding ratio (G-zratio) of from 0.7 at 6,000

s.f.p.m. to about 2.3 at 2,200 s.f.p.m. In addition, it was found that the surface chemical reaction between the titanium and the abrasive could be vastly affected by certain grinding fluids, which was not the case at the previous higher operating speed of 6,000 s.f.p.m. By using certain water soluble oil-base grinding fluids, or the so-called rust inhibitor type, G-ratios as high as 7 could be obtained. These problems and the progress made toward overcoming them can be found in more detail in a paper published by Dr. L. P. Tarasov, in American Machinist, 96,p. l35-146,(19 52).

The current state of the art is such that after some experience and experimentation with grinding wheel specifications, wheel speeds, and grinding fluids, G-ratios as high as l5 to 20 can be realized grinding titanium and its alloys. However, despite these major advancements, the present wheelgrinding conditions-grind fluid combination does have one serious limitation; to maintain a G-ratio of over 10, the infeed rate, which is a major factor in the rate at which material is removed, must be limited to 0.001 inch. This infeed limitation has been found to be the result of shortcomings in the grinding fluids currently in use, i.e. the highly chlorinated water soluble oil-based fluids and the rust inhibitor types mentioned above, the latter type being made up of 5-10 percent sodium or potassium nitrite in water. For example, when grinding a titanium alloy using a wheel specification suitable for the ap plication, a 10 percent sodium nitrite in water grinding fluid and so-called optimum grinding conditions, one of which is an infeed of 0.001 inches, a G-ratio of about 20 is realized. When the identical titanium alloy is ground with the same wheel, the same grinding fluid, and the same grinding conditions with the exception that the infeed is increased to 0.002 inches, the G- ratio then drops drastically to about 6.9 as the result of a very severe increase in wheel wear rate, which makes the overall grinding process with a 0.002 inches infeed less efficient and thereby less economical than the grinding process wherein an infeed rate of 0.001 inches is used.

In almost all grinding operations, it is highly desirable to remove metal at the fastest rate possible, this having the obvious effect of reducing the cost of the overall grinding job particularly in view of the high labor and overhead costs of present times. However, little or nothing is gained if the increase of stock removal is acquired at the price of a wheel wear rate so high as to offset or cancel the benefit of the increase in stock removal rate.

SUMMARY OF THE INVENTION The present invention is a novel grinding fluid composition which allows precision grinding processes used to grind titanium metal and titanium metal alloys, to be performed at at least twice the metal removal rate possible with currently known grinding fluids when the grinding wheels contain aluminoustype abrasive. In addition, the invention results in a 20 percent improvement in G-ratio when theinvention fluid is used with the standard acceptable infeed rate of 0.001 inches. The invention grinding fluid consists essentially of an aqueous solution of an inorganic salt of nitrous acid and an inorganic salt of formic acid which will dissolve in water to provide nitrite and formate ions. The inorganic salts of the two acids may be any nitrite and formate which are soluble in cold water and are not toxic to humans; barium salts are salts which though operative, are of questionable utility because water soluble barium salts have been known to be toxic. The cation portion of the two salts may be the same, for example sodium nitrite-sodium formate, or may be diflerent, for example potassium nitritecalcium formate, or may be mixed salts like calcium nitrite and barium nitrite-potassium formate and calcium formate.

In the course of an investigation of grinding fluids for use in the precision grinding of titanium and its alloys, with aluminum oxide type abrasives, it was discovered that one of the best known grinding fluids, viz an aqueous solution of sodium nitrite, could be significantly improved upon by additions of certain quantities of formate ion. This improvement was surprisingly manifested in the following two ways, (a) the inherent grinding quality expressed as G-ratio as defined above, was improved, and (b) the precision grinding of titanium could be accomplished at a more rapid rate because the invention fluid allowed a much higher infeed rate; the significance of the latter is that an increased infeed rate, other things being constant, means an increase in metal removal rate.

The significance of G-ratio and of infeed rate is well understood by those skilled in the art of precision grinding. The G-ratio, as defined above, is the cubic inches of material removed divided by the cubic inches of wheel wear; knowing the cost of a grinding wheel and the amount of usable abrasive therein, the grinding wheel costs for removing any given quantity of metal can be readily calculated. In addition to wheel costs, the wheel user also has a major interest in the rate of metal removal, or in precision grinding the infeed, which governs the amount of time required to remove a given unit volume of metal and therefore the labor costs. For example, in a given precision grinding operation an infeed per pass of 0.001 inches would result in a labor cost of X dollars to remove Y cubic inches of metal; if the infeed per pass in this grinding operation could be increased to 0.002 inches per pass, the labor costs for removing Y cubic inches of metal resulting from doubling the rate of metal removal, would then be x/2.

When the highly effective grinding fluid consisting of percent sodium nitrite in water was compared to the nitriteformate-water fluid of the instant invention, the latter resulted in a 20 percent to 35 percent improvement in G-ratio grinding a titanium alloy, the composition of which was 6 percent aluminum, 6 percent vanadium, 2 percent tin, and 86 percent titanium, hereinafter referred to as Ti-6,6,2. The effect of the incorporation of the formate ions can be readily seen by reference to table I. An increase in G-ratio of from 20with the sodium nitrate based grinding fluid to 23.9 for the sodium nitrite-sodium formate based fluid, with the same infeed (metal removal) used in both cases, represents an approximate 20 percent decrease in wheel costs, and the use of potassium nitrite-sodium formate fluid system results in a 35 percent improvement over the straight sodium nitrite based fluid.

Wheel: Norton Company vitrified bonded alumina abrasive wheel, having the specification: 32A60-M5VBE Wheel Speed: 1,500 s.f.p.m.

Table Feed: 400 in./min.

Downfeed: 0.001

Cross Feed: 0.050"

Metal: Ti-6,6,2

The second manifestation of the superiority of the invention grinding fluids, which is at least potentially the more important of the two, was the higher infeed rates permissible with the said invention fluids as compared to the maximum permissible infeed rates provided by the currently used sodium nitrite based fluids. An analysis of the data contained in table II shows the extreme effect that increasing the infeed had on the G-ratio. When grinding Ti-6,6,2 using the sodium nitrite based grinding fluid, the increase in infeed from 0.001 to 0.002 inches produced an extremely severe increase in wheel wear as evidenced by the decrease in G-ratio of from 20.0 to 6.9 respectively for the 0.001 inches and 0.002 inches infeeds. By contrast, when the same infeed change was made using a grinding fluid made up of 5 percent by weight of sodium nitrite and 5 percent by weight of sodium formate, the subsequent increase in wheel wear was much less severe as indicated by the change in G-ratio offrom 23.9 at a 0.001 inches infeed to 14.8 for the 0.002 inches infeed.

Grinding Conditions: Same as in Table I.

The 14.8 G-ratio using the sodium nitrite-sodium formate based grinding fluid was the result of an amount of wheel wear per unit volume of metal removed that was less than k the corresponding wheel wear expressed in the 6.9 G-ratio when the sodium nitrite based grinding fluid was used. This means that when using the fluid of this invention, the wheel cost per unit volume of metal removed is only about of the wheel cost when the same grinding operation is done using the more conventional sodium nitrite based fluid.

The nitrite-formate based grinding fluids are relatively insensitive to variations in concentration of either the total salt concentration or the relative proportions of nitrite to formate. The G-ratios contained in table III, for fluids varying in total salts concentration of from 10 to 20 weight percent, were little affected by this concentration variation. Similarly, variations in the relative amounts of formate to nitrite had little effect on the G-ratio.

Table III Composition of Aqueous Sol.-Wt.% Specific Nitrite- Grinding Conditions: Same as in Tables I and II.

Generally, the optimum total salts concentration is believed to be about percent by weight for the precision grinding of most titanium alloys under most grinding conditions. How ever, variations in the titanium alloy, type of machine, desired infeed or table traverse speed, grinding wheel types and operating speed, or the like, could conceivably make a higher or lower concentration than 10 percent more desirable. The nitriteformate grinding fluids are effective in as low a concentration of total salts as 0.5 percent by weight, and in a weight ratio of either anion to the other anion of from about 0.2 to 5.0. The optimum concentration for any specific set of conditions is easily ascertained by a minor amount of experimentation, or one can with confidence, use an aqueous solution containing 5 percent by weight of the nitrite and 5 percent by weight of the formate. The preferred salts are the nitrites and formates of potassium, sodium and calcium.

The reasons for the marked improvements resulting from the introduction of formate ions into nitrite based grinding fluids is not completely understood. It seems reasonable that the theory propounded by M. C. Shaw and C. T. Yang, in their paper, Inorganic Grinding Fluids for Titanium Alloys, Transactions of the ASME, 78, 861 (1956), can be used to at least explain the overall effect of nitrite-formate grinding fluids as well as those based on the nitrite alone. Shaw and Yang show that aqueous solutions of inorganic salts behave as they do when used as grinding fluids for grinding titanium and its alloys, because the cations and anions become adsorbed on the surfaces of the abrasive and titanium respectively, and so located, prevent bonds from forming between the abrasive and the metal which when ruptured cause rapid abrasive wear. The effectiveness of the adsorbed ion layer is controlled to a considerable degree by the ability of the adsorbed ion to closely fit the ions of the surface, and the effective charge per ion. It

is difficult, however, to apply this theory to explain the improvements realized when, for example, 30 percent of the nitrite ions in a 10 percent solution of sodium nitrite in water, is replaced by formate ions. None the less, despite the inability to explain the phenomenon, the fact remains, that the presence of formate ions in a solution containing nitrite ions, manifests itself in a superior grinding fluid for titanium and its alloys.

What is claimed is:

1. An aqueous grinding fluid suitable for grinding titanium and its alloys with aluminum oxide or aluminum oxide containing abrasives, said grinding fluid containing a total weight of from 0.5 to 20 percent of combined water soluble salts, said combined salts being made up of at least one metal salt of nitrous acid and at least one metal salt of formic acid, with the weight ratio of the anions of said salts being in the range of 0.2 to 5 .0.

2. The aqueous grinding fluid of claim 1 wherein the said total weight of said combined salts is from 5 percent to 20 percent.

3. The aqueous grinding fluid of claim ll wherein the said salts are metal salts selected from the group consisting of calcium nitrite, sodium nitrite, potassium nitrite, calcium formate, sodium formate, and potassium formate.

4. The aqueous grinding fluid of claim 3 wherein the said total weight of said combined salts is from 5 percent to 20 percent.

5. A method or grinding titanium and titanium alloys with aluminum oxide or aluminum oxide containing abrasives using an aqueous grinding fluid containing a total weight of from 0.5 to 20 percent of combined water soluble salts, said combined salts being made up of at least one metal salt or nitrous acid and at least one metal salt of formic acid, with the weight ratio of the anions of said salts being in the range of 0.2 to 5.0. 

2. The aqueous grinding fluid of claim 1 wherein the said total weight of said combined salts is from 5 percent to 20 percent.
 3. The aqueous grinding fluid of claim 1 wherein the said salts are metal salts selected from the group consisting of calcium nitrite, sodium nitrite, potassium nitrite, calcium formate, sodium formate, and potassium formate.
 4. The aqueous grinding fluid of claim 3 wherein the said total weight of said combined salts is from 5 percent to 20 percent.
 5. A method or grinding titanium and titanium alloys with aluminum oxide or aluminum oxide containing abrasives using an aqueous grinding fluid containing a total weight of from 0.5 to 20 percent of combined water soluble salts, said combined salts being made up of at least one metal salt or nitrous acid and at least one metal salt of formic acid, with the weight ratio of the anions of said salts being in the range of 0.2 to 5.0. 